1. Field of the Invention
The present invention relates to a process for recovering olefins from gaseous mixtures and, in particular, relates to the recovery of ethylene and propylene from a gaseous mixture comprising ethylene, propylene, saturated hydrocarbons and optionally other olefins.
2. Description of the Related Art
The selective absorption of ethylene in copper salt solutions is described by F Asinger (translated by B J Mazzard) in Mono-olefins, Chemistry and Technology, Pergamon Press, 1968, at pages 256 to 259. This also compares the solubility of ethylene with the solubilities of other olefins such as propylene in copper (I) chloride-ethanolamine solutions. A process for the separation of ethylene from the dehydrogenation products of ethane is described. This process is the subject of GB 428,106.
U.S. Pat. No. 2,245,719 relates to absorption of lower olefins (ethylene, propylene and butylenes) from gaseous mixtures containing the olefins and saturated hydrocarbons by contacting the gaseous mixture with cool solutions of cuprous is salts and liquid organic nitrogen compounds such as pyridine, piperidine, formamide and acetamide, preferably pyridine. The solubility of the saturated hydrocarbons and hydrogen is low in this absorbent solution. A substantially pure ethylene gas is said to be obtainable from the olefin saturated absorption solution by partially releasing the pressure or slightly heating to first evolve the propylene with a small amount of ethylene, after which an ethylene-rich gas can be obtained on further heating or lowering of the pressure.
EP 0038077 relates to a process in which a monoolefin can be separated from another monoolefin by contacting a mixture of the olefins with a complexing agent selected from cuprous salts of sulfonic acids or dialkyl phosphates dissolved in a suitable hydrocarbon solvent under conditions such that the monoolefins form complexes of different strengths with the complexing agent. The use of a suitable hydrocarbon solvent is said to be critical and aqueous solutions of cuprous salts are said to lack stability and be generally unsuitable for forming the complexing agent. According to EP 0038077 any monoolefin can be separated from another monoolefin so long as the two monoolefins form complexes of different strengths with the complexing agent. In particular, isomers of butene or isomers of pentene are said to be separable. It is said to be more difficult to separate isomers of hexene or heavier olefins. Table I of EP 0038077 sets forth the equilibrium constants (K values) for olefins with a copper (I) dodecylbenzene sulfonate in p-xylene complexing reagent. According to EP 0038077, the Table shows that the process would be very effective in separating cis-butene-2 from trans-butene-2, which have K values of 7.53 and 2.69 respectively. According to EP 0038077, the Table shows that the process would be effective for separating butene-1 from isobutene, which have K values of 6.6 and 4.74 respectively. The separation of propylene from ethylene with K values of 1.72 and 1.31 respectively, is not described in EP 0038077. According to the data in EP 0038077, the K values of ethylene and propylene are quite similar, any difference indicating that propylene forms the stronger complex.
The Proceeding of the Sixth World Petroleum Congress, Frankfurt-amo-Main, June 19-26, Section IV entitled "Base Stocks from Petroleum and Natural Gas For the Chemical Industry" published by Verein zur Forderung des 6. Welt-Erdol-Kongresses, Hamburg describes in paper 14 the recovery of mono-olefins with the aid of metal salt solutions. Two industrial processes are described; cracked gas separation with ethanolamine copper (I) nitrate solution and recovery ofolefins with silver-fluoroborate. According to the paper at pages 340 to 341, copper ethanolamine solution has a lower capacity for propylene than for ethylene so that small amounts of propylene can be removed by stripping the charged solution with ethylene.
There remains a need for a process to recover ethylene and propylene separately from a gaseous mixture comprising ethylene, propylene, saturated hydrocarbons and optionally other olefins.